Process and equipment for the continuous production of block-shaped foam, and particularly polyurethane foams wherein a liquid reaction mixture is applied to a moving bottom film and allowed to foam are known in the art. The art has generally used equipment consisting of a conveyor belt provided with side walls, and a take-off station for the bottom film as well as a device to apply the mixture to the film, located at the starting side of the belt.
Foam blocks produced according to the above technique generally have a hardened layer directly above the bottom film of a density that is 10 to 20 times higher than that of the remaining foam. The thickness of this layer, or crust, is generally between 2 and 5 mm, depending on the composition of the reaction mixture as well as on the operating conditions of the equipment. Commercially, the foam blocks produced have a height of approximately 1 meter. The volume reduction due to the hardened layer is generally only 0.2 to 0.5%. However, due to the very high density of this layer, losses of approximately 1 to 5% are incurred, based on the total amount of foam block produced. In extreme cases, these losses may amount up to 10% by weight. Generally, to determine the economic feasibility of a foam block production line, an average loss caused by the hardened layer of 3.0% by weight is assumed.
It is known that the thickness of the hardened layer can be reduced by applying the reaction mixture to the bottom film in a condition where the reaction has already started, that is, by applying an already occurring foam reaction. For this purpose, the conveyor belt travels on a downward-sloping surface. Before the conveyor belt is an overflow container into which the reaction mixture is fed from the bottom. This overflow container is provided so that the reaction mixture inside starts to react as the foam rises to the top. On top of the overflow container, the foamable mixture has already started to foam, which foam is then carried along the slanted surface and continues to foam there. Characteristic of this technique are the processes described in U.S. Pat. Nos. 3,786,122, and 3,870,441. This process has the disadvantage that gas bubbles locked into the reaction mixture can no longer escape through the surface, since the completely reacted foam has such a toughness that the gas bubbles cannot penetrate it. Therefore, the bubbles remain in the foam bun as undesired faults. Another disadvantage of this process is there is no compatibility for all the various reaction mixtures commonly used today.
The purpose of the invention is to provide foam buns with improved homogenity in the bottom area, wherein the hardened layer is entirely eliminated or, at least, where its thickness is substantially decreased.